A Direct Digital Frequency Synthesizer (DDFS) is a digitally-controlled signal generator that may vary the output signal frequency over a large range of frequencies, based on a single fixed-frequency precision reference clock. In addition, a DDFS is also phase-tunable. In essence, within the DDFS, discrete amplitude levels are fed to a Digital-to-Analog Converter (DAC) at a sampling rate determined by the fixed-frequency reference clock. The output of the DDFS provides a signal whose shape depends on the sequence of discrete amplitude levels that are fed to the DAC at the constant sampling rate. The DDFS is particularly well suited as a frequency generator that outputs a sine or other periodic waveforms over a large range of frequencies, from almost DC to approximately half the fixed-frequency reference clock frequency.
A DDFS offers a larger range of operating frequencies and requires no feedback loop, thereby providing near instantaneous phase- and frequency changes, avoiding over- and undershooting and settling time issues associated with another analog systems. A DDFS may provide precise digitally-controlled frequency and/or phase changes without signal discontinuities.
In some communication systems, inphase and quadratrue channels are modulated onto a carrier frequency that may be generated and controlled in an analog Phase-Locked Loop comprising a phase detector, a loop filter, an amplifier and a voltage controlled oscillator in a feedback-loop configuration. These analog PLLs are limited to operations within a narrow range of frequencies, sometimes referred to as the lock range. Furthermore, because of the feedback-loop configuration, any change in frequency may require the analog PLL to transition to a new steady-state. Hence, the rate of frequency changes may be limited to how fast the analog PLL may settle. For these reasons, some analog PLLs may not be suited to wideband operations with fast-switching of frequencies.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.